The wheel was lying upon the ground near, ready to receive the tire.

It was the hind wheel of a wagon.

The wagon itself was standing in front of the shop, with one end of the hind axletree supported by a block.
"What do they heat the tire for ?" asked Marco.
"To swell it," replied Forester.

"It is necessary to have the tire go on very tight, so as to hold the wheel together with all the force of the iron.

Now when iron is heated it swells, and then shrinks again when it cools.